Various steel sheets are used for a vehicle, and a steel sheet with strength over 980 MPa is used for framework components. This is to enable reduction in weight of a vehicle body to improve mileage of a vehicle while securing collision safety. Improvement in strength is required also for a steel sheet for panel components to enable both the reduction in weight of the vehicle body and the collision safety, and press formability is very important for the steel sheet for panel components depending on usage thereof.
For example, an ultralow carbon steel sheet where Ti and Nb are added is used for outer plate components where the press formability is required, and in particular, a BH (bake hardening) steel sheet is used for an outer plate panel of a door to supply a dent resistance property. Further, a steel sheet with a low yield strength is used from a viewpoint of avoiding a plane defect called as a surface strain, which occurs in press forming. As stated above, high strength as same as the framework component is not required and high ductility as same as a mild steel sheet is required due to restrictions such as the surface strain generated in press forming as for the steel sheet used for a complicated shaped component such as an inner plate or the outer plate component even for the steel sheet for the panel component.
A TRIP (transformation-induced plasticity) steel sheet where a transformation-induced plasticity effect is used is known as a steel sheet including ductility and high-strength. For example, a high-strength hot-dip galvanized steel sheet for outer plate and a method of manufacturing the same are disclosed in Patent Literature 1.
However, it is difficult for a conventional TRIP steel sheet including a steel sheet disclosed in Patent Literature 1 to obtain more excellent ductility and hole expandability while obtaining strength of 380 MPa to 630 MPa, which is suitable for a panel component. Various steel sheets are also disclosed in Patent Literatures 2 to 5, but it is difficult to obtain the more excellent ductility and hole expandability while obtaining the strength of 380 MPa to 630 MPa.